The invention relates generally to intermetallic alloy compositions such as aluminides in the form of sheets and a powder metallurgical technique for preparation of such materials.
Iron base alloys containing aluminum can have ordered and disordered body centered crystal structures. For instance, iron aluminide alloys having intermetallic alloy compositions contain iron and aluminum in various atomic proportions such as Fe3Al, FeAl, FeAl2, FeAl3, and Fe2Al5. Fe3Al intermetallic iron aluminides having a body centered cubic ordered crystal structure are disclosed in U.S. Pat. Nos. 5,320,802; 5,158,744; 5,024,109; and 4,961,903. Such ordered crystal structures generally contain 25 to 40 atomic % Al and alloying additions such as Zr, B, Mo, C, Cr, V, Nb, Si and Y.
An iron aluminide alloy having a disordered body centered crystal structure is disclosed in U.S. Pat. No. 5,238,645 wherein the alloy includes, in weight %, 8-9.5 Al, xe2x89xa67 Cr, xe2x89xa64 Mo, xe2x89xa60.05 C, xe2x89xa60.5 Zr and xe2x89xa60.1 Y, preferably 4.5-5.5 Cr, 1.8-2.2 Mo, 0.02-0.032 C and 0.15-0.25 Zr. Except for three binary alloys having 8.46, 12.04 and 15.90 wt % Al, respectively, all of the specific alloy compositions disclosed in the ""645 patent include a minimum of 5 wt % Cr. Further, the ""645 patent states that the alloying elements improve strength, room-temperature ductility, high temperature oxidation resistance, aqueous corrosion resistance and resistance to pitting. The ""645 patent does not relate to electrical resistance heating elements and does not address properties such as thermal fatigue resistance, electrical resistivity or high temperature sag resistance.
Iron-base alloys containing 3-18 wt % Al, 0.05-0.5 wt % Zr, 0.01-0.1 wt % B and optional Cr, Ti and Mo are disclosed in U.S. Pat. No. 3,026,197 and Canadian Patent No. 648,140. The Zr and B are stated to provide grain refinement, the preferred Al content is 10-18 wt % and the alloys are disclosed as having oxidation resistance and workability. However, like the ""645 patent, the ""197 and Canadian patents do not relate to electrical resistance heating elements and do not address properties such as thermal fatigue resistance, electrical resistivity or high temperature sag resistance.
U.S. Pat. No. 3,676,109 discloses an iron-base alloy containing 3-10 wt % Al, 4-8 wt % Cr, about 0.5 wt % Cu, less than 0.05 wt % C, 0.5-2 wt % Ti and optional Mn and B. The ""109 patent discloses that the Cu improves resistance to rust spotting, the Cr avoids embrittlement and the Ti provides precipitation hardening. The ""109 patent states that the alloys are useful for chemical processing equipment. All of the specific examples disclosed in the ""109 patent include 0.5 wt % Cu and at least 1 wt % Cr, with the preferred alloys having at least 9 wt % total Al and Cr, a minimum Cr or Al of at least 6 wt % and a difference between the Al and Cr contents of less than 6 wt %. However, like the ""645 patent, the ""109 patent does not relate to electrical resistance heating elements and does not address properties such as thermal fatigue resistance, electrical resistivity or high temperature sag resistance.
Iron-base aluminum containing alloys for use as electrical resistance heating elements are disclosed in U.S. Pat. Nos. 1,550,508; 1,990,650; and 2,768,915 and in Canadian Patent No. 648,141. The alloys disclosed in the ""508 patent include 20 wt % Al, 10 wt % Mn; 12-15 wt % Al, 6-8 wt % Mn; or 12-16 wt % Al, 2-10 wt % Cr. All of the specific examples disclosed in the ""508 patent include at least 6 wt % Cr and at least 10 wt % Al. The alloys disclosed in the ""650 patent include 16-20 wt % Al, 5-10 wt % Cr. xe2x89xa60.05 wt % C, xe2x89xa60.25 wt % Si, 0.1-0.5 wt % Ti, xe2x89xa61.5 wt % Mo and 0.4-1.5 wt % Mn and the only specific example includes 17.5 wt % Al, 8.5 wt % Cr, 0.44 wt % Mn, 0.36 wt % Ti, 0.02 wt % C and 0.13 wt % Si. The alloys disclosed in the ""915 patent include 10-18 wt % Al, 1-5 wt % Mo, Ti, Ta, V, Cb, Cr, Ni, B and W and the only specific example includes 16 wt % Al and 3 wt % Mo. The alloys disclosed in the Canadian patent include 6-11 wt % Al, 3-10 wt % Cr, xe2x89xa64 wt % Mn, xe2x89xa61 wt % Si, xe2x89xa60.4 wt % Ti, xe2x89xa60.5 wt % C, 0.2-0.5 wt % Zr and 0.05-0.1 wt % B and the only specific examples include at least 5 wt % Cr.
Resistance heaters of various materials are disclosed in U.S. Pat. No. 5,249,586 and in U.S. patent application Ser. Nos. 07/943,504, 08/118,665, 08/105,346 and 08/224,848.
U.S. Pat. No. 4,334,923 discloses a cold-rollable oxidation resistant iron-base alloy useful for catalytic converters containing xe2x89xa60.05% C, 0.1-2% Si, 2-8% Al, 0.02-1% Y, xe2x89xa60.009% P, xe2x89xa60.006% S and xe2x89xa60.009% O.
U.S. Pat. No. 4,684,505 discloses a heat resistant iron-base alloy containing 10-22% Al, 2-12% Ti, 2-12% Mo, 0.1-1.2% Hf, xe2x89xa61.5% Si, xe2x89xa60.3% C, xe2x89xa60.2% B, xe2x89xa61.0% Ta, xe2x89xa60.5% W, xe2x89xa60.5% V, xe2x89xa60.5% Mn, xe2x89xa60.3% Co, xe2x89xa60.3% Nb, and xe2x89xa60.2% La. The ""505 patent discloses a specific alloy having 16% Al, 0.5% Hf, 4% Mo, 3% Si, 4% Ti and 0.2% C.
Japanese Laid-open Patent Application No. 53-119721 discloses a wear resistant, high magnetic permeability alloy having good workability and containing 1.5-17% Al, 0.2-15% Cr and 0.01-8% total of optional additions of  less than 4% Si,  less than 8% Mo,  less than 8% W,  less than 8% Ti,  less than 8% Ge,  less than 8% Cu,  less than 8% V,  less than 8% Mn,  less than 8% Nb,  less than 8% Ta,  less than 8% Ni,  less than 8% Co,  less than 3% Sn,  less than 3% Sb,  less than 3% Be,  less than 3% Hf,  less than 3% Zr,  less than 0.5% Pb, and  less than 3% rare earth metal. Except for a 16% Al, balance Fe alloy, all of the specific examples in Japan ""721 include at least 1% Cr and except for a 5% Al, 3% Cr, balance Fe alloy, the remaining examples in Japan ""721 include  greater than 10% Al.
A 1990 publication in Advances in Powder Metallurgy, Vol. 2, by J. R. Knibloe et al., entitled xe2x80x9cMicrostructure And Mechanical Properties of P/M Fe3Al Alloysxe2x80x9d, pp. 219-231, discloses a powder metallurgical process for preparing Fe3Al containing 2 and 5% Cr by using an inert gas atomizer. This publication explains that Fe3Al alloys have a DO3 structure at low temperatures and transform to a B2 structure above about 550xc2x0 C. To make sheet, the powders were canned in mild steel, evacuated and hot extruded at 1000xc2x0 C. to an area reduction ratio of 9:1. After removing from the steel can, the alloy extrusion was hot forged at 1000xc2x0 C. to 0.340 inch thick, rolled at 800xc2x0 C. to sheet approximately 0.10 inch thick and finish rolled at 650xc2x0 C. to 0.030 inch. According to this publication, the atomized powders were generally spherical and provided dense extrusions and room temperature ductility approaching 20% was achieved by maximizing the amount of B2 structure.
A 1991 publication in Mat. Res. Soc. Symp. Proc., Vol. 213, by V. K. Sikka entitled xe2x80x9cPowder Processing of Fe3Al-Based Iron-Aluminide Alloys,xe2x80x9d pp. 901-906, discloses a process of preparing 2 and 5% Cr containing Fe3Al-based iron-aluminide powders fabricated into sheet. This publication states that the powders were prepared by nitrogen-gas atomization and argon-gas atomization. The nitrogen-gas atomized powders had low levels of oxygen (130 ppm) and nitrogen (30 ppm). To make sheet, the powders were canned in mild steel and hot extruded at 1000xc2x0 C. to an area reduction ratio of 9:1. The extruded nitrogen-gas atomized powder had a grain size of 30 xcexcm. The steel can was removed and the bars were forged 50% at 1000xc2x0 C., rolled 50% at 850xc2x0 C. and finish rolled 50% at 650xc2x0 C. to 0.76 mm sheet.
A paper by V. K. Sikka et al., entitled xe2x80x9cPowder Production, Processing, and Properties of Fe3Alxe2x80x9d, pp. 1-11, presented at the 1990 Powder Metallurgy Conference Exhibition in Pittsburgh, Pa., discloses a process of preparing Fe3Al powder by melting constituent metals under a protective atmosphere, passing the metal through a metering nozzle and disintegrating the melt by impingement of the melt stream with nitrogen atomizing gas. The powder had low oxygen (130 ppm) and nitrogen (30 ppm) and was spherical. An extruded bar was produced by filling a 76 mm mild steel can with the powder, evacuating the can, heating 1xc2xd hour at 1000xc2x0 C. and extruding the can through a 25 mm die for a 9:1 reduction. The grain size of the extruded bar was 20 xcexcm. A sheet 0.76 mm thick was produced by removing the can, forging 50% at 1000xc2x0 C., rolling 50% at 850xc2x0 C. and finish rolling 50% at 650xc2x0 C.
Oxide dispersion strengthened iron-base alloy powders are disclosed in U.S. Pat. Nos. 4,391,634 and 5,032,190. The ""634 patent discloses Ti-free alloys containing 10-40% Cr, 1-10% Al and  less than 10% oxide dispersoid. The ""190 patent discloses a method of forming sheet from alloy MA 956 having 75% Fe, 20% Cr, 4.5% Al, 0.5% Ti and 0.5% Y2O3.
A publication by A. LeFort et al., entitled xe2x80x9cMechanical Behavior of FeAl40 Intermetallic Alloysxe2x80x9d presented at the Proceedings of International Symposium on Intermetallic Compoundsxe2x80x94Structure and Mechanical Properties (JIMIS-6), pp. 579-583, held in Sendai, Japan on Jun. 17-20, 1991, discloses various properties of FeAl alloys (25 wt % Al) with additions of boron, zirconium, chromium and cerium. The alloys were prepared by vacuum casting and extruding at 1100xc2x0 C. or formed by compression at 1000xc2x0 C. and 1100xc2x0 C. This article explains that the excellent resistance of FeAl compounds in oxidizing and sulfidizing conditions is due to the high Al content and the stability of the B2 ordered structure.
A publication by D. Pocci et al., entitled xe2x80x9cProduction and Properties of CSM FeAl Intermetallic Alloysxe2x80x9d presented at the Minerals, Metals and Materials Society Conference (1994 TMS Conference) on xe2x80x9cProcessing, Properties and Applications of Iron Aluminidesxe2x80x9d, pp. 19-30, held in San Francisco, Calif. on Feb. 27-Mar. 3, 1994, discloses various properties of Fe40Al intermetallic compounds processed by different techniques such as casting and extrusion, gas atomization of powder and extrusion and mechanical alloying of powder and extrusion and that mechanical alloying has been employed to reinforce the material with a fine oxide dispersion. The article states that FeAl alloys were prepared having a B2 ordered crystal structure, an Al content ranging from 23 to 25 wt % (about 40 at %) and alloying additions of Zr, Cr, Ce, C, B and Y2O3. The article states that the materials are candidates as structural materials in corrosive environments at high temperatures and will find use in thermal engines, compressor stages of jet engines, coal gasification plants and the petrochemical industry.
A publication by J. H. Schneibel entitled xe2x80x9cSelected Properties of Iron Aluminidesxe2x80x9d, pp. 329-341, presented at the 1994 TMS Conference discloses properties of iron aluminides. This article reports properties such as melting temperatures, electrical resistivity, thermal conductivity, thermal expansion and mechanical properties of various FeAl compositions.
A publication by J. Baker entitled xe2x80x9cFlow and Fracture of FeAlxe2x80x9d, pp. 101-115, presented at the 1994 TMS Conference discloses an overview of the flow and fracture of the B2 compound FeAl. This article states that prior heat treatments strongly affect the mechanical properties of FeAl and that higher cooling rates after elevated temperature annealing provide higher room temperature yield strength and hardness but lower ductility due to excess vacancies. With respect to such vacancies, the articles indicates that the presence of solute atoms tends to mitigate the retained vacancy effect and long term annealing can be used to remove excess vacancies.
A publication by D. J. Alexander entitled xe2x80x9cImpact Behavior of FeAl Alloy FA-350xe2x80x9d, pp. 193-202, presented at the 1994 TMS Conference discloses impact and tensile properties of iron aluminide alloy FA-350. The FA-350 alloy includes, in atomic %, 35.8% Al, 0.2% Mo, 0.05% Zr and 0.13% C.
A publication by C. H. Kong entitled xe2x80x9cThe Effect of Ternary Additions on the Vacancy Hardening and Defect Structure of FeAlxe2x80x9d, pp. 231-239, presented at the 1994 TMS Conference discloses the effect of ternary alloying additions on FeAl alloys. This article states that the B2 structured compound FeAl exhibits low room temperature ductility and unacceptably low high temperature strength above 500xc2x0 C. The article states that room temperature brittleness is caused by retention of a high concentration of vacancies following high temperature heat treatments. The article discusses the effects of various ternary alloying additions such as Cu, Ni, Co, Mn, Cr, V and Ti as well as high temperature annealing and subsequent low temperature vacancy-relieving heat treatment.
A publication by D. J. Gaydosh et al., entitled xe2x80x9cMicrostructure and Tensile Properties of Fe-40 At.Pct. Al Alloys with C, Zr, Hf and B Additionsxe2x80x9d in the September 1989 Met. Trans A, Vol. 20A, pp. 1701-1714, discloses hot extrusion of gas-atomized powder wherein the powder either includes C, Zr and Hf as prealloyed additions or B is added to a previously prepared iron-aluminum powder.
A publication by C. G. McKamey et al., entitled xe2x80x9cA review of recent developments in Fe3Al-based Alloysxe2x80x9d in the August 1991 J. of Mater. Res., Vol. 6, No. 8, pp. 1779-1805, discloses techniques for obtaining iron-aluminide powders by inert gas atomization and preparing ternary alloy powders based on Fe3Al by mixing alloy powders to produce the desired alloy composition and consolidating by hot extrusion, i.e., preparation of Fe3Al-based powders by nitrogen- or argon-gas atomization and consolidation to full density by extruding at 1000xc2x0 C. to an area reduction of xe2x89xa69:1.
U.S. Pat. Nos. 4,917,858; 5,269,830; and 5,455,001 disclose powder metallurgical techniques for preparation of intermetallic compositions by (1) rolling blended powder into green foil, sintering and pressing the foil to full density, (2) reactive sintering of Fe and Al powders to form iron aluminide or by preparing Nixe2x80x94Bxe2x80x94Al and Nixe2x80x94Bxe2x80x94Ni composite powders by electroless plating, canning the powder in a tube, heat treating the canned powder, cold rolling the tube-canned powder and heat treating the cold rolled powder to obtain an intermetallic compound. U.S. Pat. No. 5,484,568 discloses a powder metallurgical technique for preparing heating elements by micropyretic synthesis wherein a combustion wave converts reactants to a desired product. In this process, a filler material, a reactive system and a plasticizer are formed into a slurry and shaped by plastic extrusion, slip casting or coating followed by combusting the shape by ignition. U.S. Pat. No. 5,489,411 discloses a powder metallurgical technique for preparing titanium aluminide foil by plasma spraying a coilable strip, heat treating the strip to relieve residual stresses, placing the rough sides of two such strips together and squeezing the strips together between pressure bonding rolls, followed by solution annealing, cold rolling and intermediate anneals.
U.S. Pat. No. 4,385,929 discloses a method for making irregularly shaped steel powder with low oxygen content by an atomizing technique wherein a molten stream of metal is contacted with a non-polar solvent such as mineral oil, animal or vegetable oil.
U.S. Pat. No. 3,144,330 discloses a powder metallurgical technique for making electrical resistance iron-aluminum alloys by hot rolling and cold rolling elemental powder, prealloyed powders or mixtures thereof into strip. U.S. Pat. No. 2,889,224 discloses a technique for preparing sheet from carbonyl nickel powder or carbonyl iron powder by cold rolling and annealing the powder.
Based on the foregoing, there is a need in the art for an economical technique for preparing intermetallic compositions such as iron aluminides. There is also a need in the art for an economical technique for preparing resistance heating elements from intermetallic alloy compositions such as iron aluminides which exhibit a desirable resistivity at an aluminum concentration which heretofore has required hot working steps such as extrusion of canned FeAl powder/cast metal or hot rolling of clad FeAl powder/cast metal. For instance, conventional powder metallurgical techniques of preparing iron-aluminides include melting iron and aluminum and inert gas atomizing the melt to form an iron-aluminide powder, canning the powder and working the canned material at elevated temperatures. It would be desirable if iron-aluminide could be prepared by a powder metallurgical technique wherein it is not necessary to can the powder and wherein it is not necessary to subject the iron and aluminum to any hot working steps in order to form an iron-aluminide sheet product.
The invention provides a method of manufacturing a metal sheet having an intermetallic alloy composition by a powder metallurgical technique. The method includes forming a non-densified metal sheet by consolidating a prealloyed powder having an intermetallic alloy composition; forming a cold rolled sheet by cold rolling the non-densified metal sheet so as to densify and reduce the thickness thereof; and heat treating the cold rolled sheet.
According to a preferred embodiment, the intermetallic alloy is an iron aluminide alloy. The iron aluminide can include, in weight %, 4.0 to 32.0% Al and have a ferritic microstructure which is austenite-free. The intermetallic alloy can comprise Fe3Al, Fe2Al5, FeAl3, FeAl, FeAlC, Fe3AlC or mixtures thereof. The iron aluminide can comprise, in weight %, xe2x89xa62% Mo, xe2x89xa61% Zr, xe2x89xa62% Si, xe2x89xa630% Ni, xe2x89xa610% Cr, xe2x89xa60.5% C, xe2x89xa60.5% Y, xe2x89xa60.1% B, xe2x89xa61% Nb and xe2x89xa61% Ta. For instance, the iron aluminide can consist essentially of, in weight %, 20-32% Al, 0.3-0.5% Mo, 0.05-0.3% Zr, 0.01-0.5% C, xe2x89xa61% Al2O3 particles, xe2x89xa61% Y2O3 particles, balance Fe.
The method can include various optional steps and/or features. For instance, the consolidation step can comprise tape casting a mixture of the powder and a binder, roll compacting a mixture of the powder and a binder or plasma spraying the powder onto a substrate. In the case of tape casting or roll compaction, the method can include heating the non-densified metal sheet at a temperature sufficient to remove volatile components from the non-densified metal sheet. For instance, the article can be heated to a temperature below 500xc2x0 C. during the step of removing the volatile components.
According to a preferred embodiment, the method includes forming the cold rolled sheet into an electrical resistance heating element subsequent to the heat treating step, the electrical resistance heating element being capable of heating to 900xc2x0 C. in less than 1 second when a voltage up to 10 volts and up to 6 amps is passed through the heating element.
According to one embodiment, the non-densified metal sheet is initially or fully sintered prior to the cold rolling step and the cold rolling step can be repeated with intermediate annealing of the cold rolled sheet. The final cold rolling step can be followed by a stress relieving heat treatment. The powder can comprise gas or water or polymer atomized powder and the method can further comprise sieving the powder and in the case of roll compaction or tape casting, coating the powder with a binder prior to the consolidation step. The heat treating step can be carried out at a temperature of 1000 to 1200xc2x0 C. in a vacuum or inert atmosphere. In the final cold rolling step the sheet can be reduced to a thickness of less than 0.010 inch. The powder can have a particle size distribution of 10 to 200 xcexcm, preferably 30 to 60 xcexcm. For example, the powder used for tape casting preferably passes 325 mesh and the powder used for roll compaction preferably comprises a mixture of 43 to 150 xcexcm powder with a small amount (e.g. 5%) of  less than 43 xcexcm powder.
Due to the hardness of the intermetallic alloy it is advantageous if cold rolling is carried out with rollers having carbide rolling surfaces in direct contact with the sheet. The sheet is preferably produced without hot working the intermetallic alloy.